Descriptions

The two most economically important plant pathogens in the Pseudoperonospora (Peronosporaceae) genus are P. cubensis, causal agent of cucurbit downy mildew, and P. humuli, causal agent of hop downy mildew. These organisms have been shown to be very closely related phylogenetically and morphologically. In 2005, researchers in Korea proposed that based on morphological similarities and internal transcribed spacer of nuclear ribosomal DNA (ITS nrDNA) sequence data, P. humuli should be reduced taxonomically to a synonym of P. cubensis. As this taxonomic change has implications for identification, management, and regulation, the current study further explores this issue using multigenic analyses and host specificity experiments.
Multigenic sequence analyses were conducted considering five loci for 21 isolates of P. cubensis and 14 isolates of P. humuli. The five loci used in the analysis were the ITS, β-tubulin gene, cytochrome c oxidase II gene (cox2), cytochrome c oxidase I gene (cox1), and the spacer between cox2 and cox1. Additionally, the cytochrome c oxidase genes and spacer were combined for analysis as the cox cluster, and all five loci were concatenated for a robust analysis using Bayesian and maximum likelihood inference. Although the topology and statistical support for the topology for each locus differed, there was a consistent separation of a majority of the P. humuli isolates and the P. cubensis isolates. The primary exceptions were an isolate of P. humuli from Korea on Humulus japonicus and an isolate of P. cubensis from North Carolina on acorn squash.
Two reportedly universally susceptible hosts of P. cubensis (cucumber cv. Straight 8 and cantaloupe cv. Ananes Yokneam) were inoculated with four isolates of P. humuli from the western U.S. Two highly susceptible hosts of P. humuli (cvs. Nugget and Pacific Gem) were inoculated with eight isolates of P. cubensis from the eastern U.S. P. cubensis frequently infected the hop cultivars but at low rates (77% of replicate plants, typically with fewer than a thousand sporangia per plant) while P. humuli produced only one sporangiophore during the course of the studies (3% of replicate plants). Thus, there is evidence that biologically relevant characteristics exist that differentiate the two organisms with implications for the detection and management of both that may be concealed by the reduction of P. humuli to a taxonomic synonym of P. cubensis.